1. Field of the Invention
This invention relates to semiconductor processes, and more particularly, to processes for diffusing impurities into semiconductor material.
2. Description of the Prior Art
In the manufacture of some semiconductor device structures, it is necessary to form isolation regions that extend completely through the device to provide electrical isolation between active portions of the device. It is known to isolate areas on a semiconductor wafer by etching troughs in the wafer between devices. However, such troughs would weaken or destroy the devices if they were to extend completely through the wafer. Additionally, devices so isolated would expose the doped layers of the devices along the walls of the troughs which can result in undesirable leakage currents therebetween.
Isolation regions may also be formed in a silicon wafer by initially growing an oxide layer on at least one of the major surfaces thereof. Openings are selectively formed in the oxide layer using well-known photolithographic and etching techniques. Dopant material is then deposited on the major surfaces of the wafer, including the oxide layer openings, and driven into the wafer to a first depth of about 0.1 of a micron in a high temperature furnace at about 1200.degree. C. for about 20 minutes. The remaining dopant material is then removed and the wafer is again placed in the high temperature furnace at about 1256.degree. C. for approximately 78 hours to diffuse the dopant into the silicon to a predetermined depth of about 0.003 of an inch. The active portions of the devices are then fabricated in the wafer, between the isolation regions, in a well-known manner and the wafer particulated into a multitude of discrete devices.
Although such a method has been found to be most effective and reliable, the length of the diffusion "drive-in" time required to form such deep isolation regions is clearly undesirable and the lateral diffusion of the dopant material is substantial (i.e., equal or greater than the depth), resulting in fewer active devices for a given wafer surface area.
Additionally, at present, such semiconductor devices, requiring deep isolation regions, are fabricated on wafers that are approximately 1.5 inches in diameter and are 0.006 of an inch thick. Conversion to larger diameter wafers would be more economical, but would necessitate an increase in the wafer thickness to provide the mechanical strength required in handling the wafers during further processing steps. However, such an increase in thickness would substantially increase the dopant diffusion time. For example, to convert from a 1.5 to a 2.0 inch wafer, it would be necessary to increase the wafer thickness to 0.010 of an inch which, in turn, would increase the dopant drive-in time from 78 hours to about 300 hours at the same temperature. Furthermore, the lateral spreading of the dopant will also increase which can interfere with the device performance as well as decreasing the number of devices that can be fabricated on a wafer.
An article entitled "Studies of Anomalous Diffusion of Impurities in Silicon, " by K. H. Nicholas in Solid-State Electronics, Pergamon Press 1966, Vol. 9, pp. 35-47 indicates that fast diffusion of impurities was found to occur in silicon crystal if the crystal had been mechanically polished. However, such mechanical polishing cannot be accomplished selectively and there is no indication that such a process could effectively decrease the lateral diffusion of the dopant material.
Accordingly, there exists a need for a process for doping semiconductor material with decreased "drive-in" times as well as decreased lateral diffusion.